In cellular telephones, such power amplifiers are intended to generate and supply the antenna with an operating power adequate for the terminal to be capable of communicating with the nearest base station. According to the GSM (Global System for Mobile Communications) standard, for example, the power amplifier generates the power needed by the antenna per 2-dB step.
FIG. 1 shows the general operation of such a power amplifier 11 in the general diagram of a cellular telephone. As shown in FIG. 1, the power amplifier 11 generates the power needed by the antenna 10, via an antenna switch 110, which makes it possible to select the frequency band and the operating mode (transmission or reception). Without describing this figure in greater detail, it can be noted that the power amplifier 11 interacts with a block 12 performing filtering and matching operations, and a block 14 performing transmission/reception functions. The baseband portion 13 was not detailed in FIG. 13. The blocks 12 to 14 of FIG. 1 are not an integral part of this invention, and are not therefore described in greater detail. For additional information, reference can be made to the conventional diagrams of radiocommunication terminals, as recommended, for example, by GSM or UMTS (Universal Mobile Telecommunication System) standards.
As shown in FIG. 1, such a power amplification device 11 includes a module 111 for controlling the power amplifier 112.
Such a control module 111 serves to control the power supplied at the output of the power amplification device 11, in particular according to the operating temperature, the supply voltage of the cellular telephone battery, the load impedance, and so on.
Today, a number of techniques for producing such a control module 111 are known, and are shown in FIGS. 2A to 2C.
The first of these techniques, shown in FIG. 2A, consists of producing a system for closed-loop control of the power supplied.
The power amplification device of FIG. 2A includes an amplifier 21, which, in a specific embodiment of the invention, can be preceded and followed by optional coupling capacitors 20, 22. The control loop of the power supplied at the output 30, intended for the antenna of the radiocommunication terminal, includes:                a coupler 23;        a module 24 for detecting the RF power, also including a comparator, also called a “detector/comparator”;        a polarisation controller 25.        
The ramp 26 supplies the detector/comparator 24 with a reference voltage, coming from the baseband 13. The coupler 23 samples some of the power (RF or microwave) supplied by the amplifier 21, and transmits it to the detector/comparator 24, which generates a voltage based on this measured power. The latter then compares the voltage that it has generated with the reference voltage supplied by the baseband 26. If the power supplied differs from the reference power (associated with the reference voltage supplied by the baseband 26), the polarisation controller 25 then modifies the voltage supplied to the amplifier 21, so as to adjust the power supplied at the output 30.
Two so-called “open-loop” techniques for controlling the power supplied at the output of such a power amplifier are also known, and are shown in FIGS. 2B and 2C.
The assembly of FIG. 2B provides control of the current power amplifier 21. Again, in the specific alternative of FIG. 2B, such an amplifier 21 is preceded and followed by two optional coupling capacitors 20, 22. The assembly of FIG. 2B includes, as above, a ramp 26 providing a reference from the baseband 13, a comparator 27 and a polarisation controller 25. The voltages Vbat and Vcc correspond respectively to the voltage supplied by the battery of the radiocommunication terminal and the voltage supplying the power amplifier 21.
Knowing the value of the resistance 28, the intensity of the current I passing therethrough can be deduced. After a comparison 24 of this intensity with the reference intensity 26, the polarisation controller 25 corrects the power amplifier set value 21, so as to adjust the power supplied at the output 30.
The assembly of FIG. 2C shows the final known technique, consisting of controlling the supply voltage of the amplifier 21 in an open loop. Again, in the specific embodiment of FIG. 2C, a first optional coupling capacitor 20 precedes the amplifier 21, and a second optional coupling capacitor 22 follows it. A MOSFET transistor 29 is used to control the supply voltage Vcc of the amplifier 21. This control is achieved by means of a comparator 27, of which the set value is given by a ramp 26 connected to the baseband 13. The voltage Vcc is controlled so that the power RF or microwave supplied at the output 30 of the amplifier is equal to the reference power associated with the reference voltage given by the ramp 26.
These various techniques of the prior art have a number of disadvantages.
Although the closed-loop control technique of FIG. 2A provides excellent control of the output power, while having a satisfactory current consumption, has the disadvantage of inducing RF losses on the order of 0.2 to 0.3 dB. However, such a system performs very well when the load varies, i.e. when the antenna is of mediocre quality.
The closed-loop control of FIG. 2A is therefore the highest-performing technique of the three prior art techniques described above, but it is also the most expensive technique. Moreover, the technologies used for the design of the couplers are different from those used for the design of amplifier chips 21 or controller chips 25 (conventionally AsGa or CMOS), which leads to major integration problems.
The open-loop voltage control of FIG. 2C has mediocre performance, in terms of both power control and current consumption, when the power supplied is low. Although it would be easier to integrate such a device in the case of FIG. 2A, this technique also has the disadvantage of a voltage drop through the MOSFET transistor, due to the existence of a parasitic resistance, which leads to reduced efficiency.
Finally, the open-loop current control technique of FIG. 2B has improved performances compared to the technique of FIG. 2C in terms of integrability, current consumption and power loss. However, it does not allow for power control as effective as that of the closed-loop technique of FIG. 2A.